From a simple and exact approach, we investigated the thermodynamic behavior of short HP-polymers (composed of hydrophobic and polar coarse-grained monomers) on the square lattice, for lengths N < 20 residues. In particular, we analyzed the heat capacity excitation peak, showing the strong correlation between its occurrence, at low temperatures, and a sharp conformational gap from ground states to first excited states (
Ω
1
/
Ω
0
≫
1
). This relation is observed for both homopolymers and heteropolymers.